The demands on digital subscriber lines have increased over the recent years, due to the development and increased use of services involving video, voice and data. Such highly demanding services are also referred to as Triple Play (3P). For example, a 3P scenario may be a combination of VoIP (Voice over Internet Protocol), IPTV (Internet Protocol TeleVision) and high speed internet access. The provision of such 3P services is a challenging task in terms of Quality of Service (QoS). For example, parasitical effects in the physical layer, such as non-transient and transient noise, which until now have been negligible, suddenly have a considerable negative effect on the transport of services over the subscriber lines.
Impulse Noise Protection (INP) is one of the features available for interference mitigation in DSL technology. INP provides, as the name suggests, means to deal with non-stationary impulse-like noise patterns, by combining Forward Error Correction (FEC) and Interleaving, thereby enhancing the lines' ability to withstand impulse-like bursts of symbol errors. INP is measured in terms of amount of consecutive symbols protected. An example of impulse-like bursts 204b is illustrated in FIG. 2b, where the SNR margin is illustrated by the line 202b. INP is controlled by the parameters Minimum INP and Maximum Delay. Assigning high values to these parameters implies a higher protection against impulse noise, at the cost of lower data rates and longer delays to subscribers. A high value of max delay would typically result in a higher bit rate, since the modem then can use more interleaving instead of Reed-Solomon redundancy bits to achieve the configured INP.
Another method for reducing the effects of DSL line interference is the selection of an appropriate SNR margin. The SNR margin provides protection against longer periods of interference such as crosstalk, i.e. non-impulse like interference. An example of non-impulse-like bursts 204a is illustrated in FIG. 2a, where the SNR margin is illustrated by the line 202a. The SNR margin allows for extra protection against noise disturbance by inducing the modems to use more conservative modulation settings. The SNR margin is set in terms of a minimum SNR margin, a target SNR margin and a maximum SNR margin value. An increase of the SNR margin provides more protection but lower bit rates. Further, trellis coding is an available feature, which is frequently used to reduce noise effects in DSL systems.
The task of finding an adequate configuration with a proper setting of the INP and SNR margin is challenging, since different settings influence the line performance in terms of throughput and bit error rate, in a non-linear fashion. Theoretically, the choice of INP settings and SNR margin could be varied in an infinite number of ways. However, in order to keep the complexity of line management on a reasonable level, most operators limit the number of possible configurations by only allowing a certain number of predefined alternatives called configuration profiles. Each allowed profile comprises a certain INP setting, a certain target SNR margin and a maximum delay value. FIG. 1 shows a schematic view of an exemplary set of allowed configuration profiles 102 in the SNR-INP plane. The profiles in FIG. 1 are placed in a grid, which is, however, not necessary. The profiles could be defined in any constellation according to e.g. experience or preference.
The values in a configuration profile are threshold values, which may be used for configuring a DSL line. These set values are, however, not necessarily the same as the actual values after configuration, since the decision of, e.g., which actual e.g. FEC block length, redundancy and interleaver depth that should be used are made in the modems. This makes the task of finding an adequate configuration even harder, as compared to in systems like e.g. DOCSIS (Data Over Cable Service Interface Specification), i.e. broadband over coaxial cables, where parameters such as FEC block length, redundancy and interleaver depth are controllable, i.e. are imposed on the modems.
One approach currently employed by many telecom operators, in order to cope with disturbances in the DSL line, is to adopt default line protection settings, either optimistic or overprotective, and change these settings manually upon subscriber complaint. This approach has been reasonably suitable for traditional data services, such as pure best effort Internet services. However, for more demanding and sensitive services, such as IPTV, this approach is unlikely to scale. For example, subscribers of an IPTV service expect high-quality video delivery, having at least the same quality level as that of cable and satellite TV. Experience shows that even small disturbances in transmission are sufficient to cause perceivable video degradation. Thus, for many 3P services, using optimistic settings of INP and SNR margin will result in insufficient quality levels, while overprotective settings of INP and SNR margin will result in insufficient bit rates.
Different solutions for automatic adaption of a line configuration to current line conditions have been suggested. For example, a method for automatic line configuration adaptation is described in the patent document US2009/0175199 [6]. Document [6] concerns a solution to the problem of monitoring a DSL line and selecting transmission settings, i.e. a configuration profile, in order to maximize rate under varying noise environments. The method described in [6] involves trying, one at a time, in the event of performance degradation, a change in INP settings and a change in target SNR margin, evaluating the result of the respective tests, and selecting the alternative which gave the best result in terms of bit rate as the next line configuration. The performance degradation may be identified e.g. via thresholds on DSL error metrics.
Other examples of automatic line configuration adaptation are described in patent documents U.S. Pat. No. 7,302,379 [7], U.S. Pat. No. 7,315,573 [8] and U.S. Pat. No. 7,035,249 [9]. Document [7] addresses the problem of DSL line optimization in a broad sense, specifying statistics collection and analysis. Document [8] concerns a method for DOCSIS, where all information necessary for selecting an optimal next configuration is assumed to be available. U.S. Pat. No. 7,035,249 [9] focuses on the timing of a reinitialization, i.e. reconfiguration, with an ambition to reduce the negative effects of a reinitialization by selecting the appropriate timing. The decision to make a reinitialization is made based on a comparison between current and historic monitored parameters.
One problem with automated line optimization solutions as the one suggested in [6], is that the different candidate line configurations need to be empirically tested in order to find a preferred candidate. For each candidate that is to be tested, the service on the line needs to be stopped and restarted in order to resynchronize the line. Each such resynchronization, or reconstruction, makes the line unavailable for tens of seconds up to minutes, depending on type of system etc. In some automatic systems, all predefined candidate configuration profiles are tested per default before selecting an appropriate configuration profile for use in reconfiguration.
The repeated interruption of the service on a line, due to resynchronizations during the search for a suitable candidate line configuration is thus identified as a problem.